The invention disclosed and claimed herein generally pertains to an RF coil arrangement for a magnetic resonance (MR) imaging system or scanner. More particularly, the invention pertains to an arrangement wherein the RF coil of the MR scanner is selectively mounted in relation to other components of the scanner, to substantially reduce the noise or acoustic disturbance which is experienced by a patient located within the bore of the scanner, that is, within the MR imaging volume.
As is well known, an MR imaging system or scanner commonly includes a cryostat, which contains a powerful superconductive main magnet positioned around a main magnet bore. The superconductive magnet is maintained at an extremely cold temperature and produces a strong static magnetic field, or B.sub.0 field, within the bore, the B.sub.0 field being directed along the bore axis. Other essential components of the MR system include the RF coil, or RF antenna, and the gradient coil assembly, which comprises a hollow cylindrical structure. The RF coil may be operated in a transmit mode, to generate MR signals in an imaging subject, or may be operated in a receive mode to detect the MR signals. The gradient coil assembly comprises one or more cylindrical coil forms, as well as a set of gradient coils supported thereby, to produce the X-, Y-, and Z-gradient magnetic fields. These fields are required to spatially encode MR data. Typically, the gradient coil assembly is positioned within the main magnet bore.
In the past, it has been common practice to support the RF coil within the main magnet bore by attaching it to a further essential MR system component comprising an inner cylindrical form. The inner form comprises a tubular member which is inserted through the gradient coil assembly, in coaxial relationship therewith. The interior region of the inner tubular member generally comprises the patient bore or imaging volume of the associated MR system, that is, the volume which is disposed to receive a patient, and in which MR signals are generated and detected. The ends of the inner tubular member are attached to the cryostat, by means of end caps or the like, so that the tubular member is supported thereby. Typically, the RF coil is placed around the outside diameter of the inner tubular member, in close adjacent relationship, and supported or carried thereby. The tubular member is made of a non-electrically conductive material, so that it does not impede RF performance within the imaging volume.
Prior art arrangements of the above type and components thereof are described, for example, in U.S. Pat. Nos. 5,570,021 and 5,760,584, both commonly assigned herewith to the General Electric Company. Two of the inventors named in U.S. Pat. No. 5,570,021 are coinventors herein. In such arrangements, the RF coil is supportably mounted on the inner form or tubular member, as a convenient technique for providing an essential spacing between the RF coil and the gradient coil assembly. However, an MR imaging system employs electrically excited gradient coils to impose time varying magnetic fields on the primary or B.sub.0 magnetic field. These time varying fields tend to induce eddy currents in the conductors of the RF coil, which in turn may cause mechanical motion of the RF coil. In the prior art arrangement described above, the inventors have recognized that such mechanical motion may be a principal reason for the occurrence of noise within the imaging volume. More specifically, the inventors have recognized that if the RF coil is joined to and supported upon the inner tubular member, the eddy current induced motion of the RF coil may drive the tubular member in the manner of an acoustic loudspeaker. The inner tubular member thus serves as a noise generator within the imaging volume. In the past, those of skill in the art tended to overlook RF coil motion as a major source of noise. It is likely that this occurred because the quality of MR images, acquired in the presence of RF coil motion caused by the induced eddy currents, did not seem to be adversely affected by such motion.